Do not do it unless you have the original author's permission. I mean, seriously.
The thing about stories is that they're very, very personal. Whatever you want to write in continuation, it is almost certainly not what the original author was going to do. And the thing about authors is that they're very, very possessive. After all, s/he owns these characters, is the ultimate arbiter of what they would or wouldn'tdo. That's why you want permission: taking the story in your personal direction, even well-meant, can be offensive at best and insulting at worst.
And if you don't get that permission, don't write a continuation. Period.
"Haiku are easy
to write, but may not make sense.
"The plural of 'Surgeon General' is 'Surgeons General.' The past tense of 'Surgeons General' is 'Surgeonsed General.' "
Picture is not me; can be found at ShyAngela.com
Here is my story site
, which has more material on it than my Literotica page
. Oh, and, I'm now on Facebook